1. Field of the Invention
The present invention relates to imaging cells and, more particularly, to a method of estimating electrical cross talk in an array of imaging cells.
2. Description of the Related Art
A color solid-state image sensor has a large number of imaging cells that are arranged in rows and columns, and a color filter that lies over the imaging cells. When exposed to light, the photons which pass through the color filter are absorbed by the imaging cells which, in turn, generate electron-hole pairs. Some of the photogenerated electrons can diffuse laterally a few cells, usually not more than two, before being collected by an adjacent imaging cell.
This lateral diffusion of electrons, known as electrical cross talk, causes color-shifting problems among image cells (pixels) in color solid-state image sensors. As a result, the captured view of monochromatic light shifts from its true color due to the addition of other color components.
In addition, the problem of electrical crosstalk becomes worse as the pitch between imaging cells shrinks (e.g., less than 5 um). The magnitude of the cross talk is a function of the doping profile, the charge free running distance, and the layout structure. For small pixels (e.g. 3.2 um pitch pixel with 0.18 um advanced CMOS processing), cross talk of red light can be as high as 20%, which causes a very bad color shifting problem that results in failed products.
A number of studies have been reported utilizing a laser-scanning technique to characterize a modulation transfer function (MTF) that is related to cross talk in CMOS image sensors. (See O. Yadid-Pecht, “The Geometrical Modulation Transfer Function (MTF) for Different Pixel Active Area Shapes”, Optical Engineering, Vol. 39, No. 4, 2000, pp. 859-865; I. Shcherback and O. Yadid-Pecht, “CMOS APS MTF Modeling”, IEEE Trans. On Electron Devices, Vol. 48, No. 12, 2001, pp. 2710-2715; and C. Marques and P. Magnan, “Experimental Characterization and Simulation of Quantum Efficiency and Optical Cross Talk of CMOS photodiode APS”, Electronic Imaging 2002, Conf. 4669A—Sensors, Cameras and Systems for Scientific/Industrial Applications IV, San Jose, Calif. 2002).
Further, cross talk and its effect on solid-state sensors and deblurring operations by employing convolution theorem are described in J. S. Lee, J. Shah, M. Ed Jernigan and R. I. Hornsey, “Empirical Characterization of Lateral Cross Talk for CMOS Image Sensors and Deblurring Operations”, IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors, Schloss Elmau, Elmau, Germany, 2003 and E. Stevens, “A Unified Model of Carrier Diffusion and Sampling Aperture Effects on MTF in Solid-State Image Sensors”, IEEE Trans. On Electron Devices, Vol. 39, No. 11, 1992, pp. 2621-2623. Micron lens study on reducing cross talk is discussed by G. Agranov, V. Berezin and R. Tsai, “Cross talk and Microlens Study in a Color CMOS Image Sensor”, IEEE Trans. On Electron Devices, Vol. 50, No. 1, 2003, pp. 4-11. All the studies are focused on either MTF or methods of reducing cross talk.
There has not been an efficient way to estimate electrical cross talk in the industry such that sources of cross talk and the amount of cross talk in different directions can be quantified and the performance of an array of imaging cells can be predicted from monochrome image sensor results. Thus, there is a need for a method of estimating cross talk in an array of imaging cells to predict the performance of the array of imaging cells.